Cemented carbide parts with cobalt, nickel, iron, or their alloys as binder material often have hard refractory coatings applied to their surfaces for improving the wear resistance of the cutting edge. Although the surface coatings are very wear resistant, they are very brittle in comparison to the sintered cemented carbide substrate and may crack during cutting. The prior art has concentrated on improving toughness or edge strength in coated cutting inserts by forming a binder enrichment of the surface to promote improved toughness and reduce the susceptibility of the cutting edge to fail by chipping or breakage.
U.S. Pat. No. 4,277,283 to Tobioka et al relates to a process cutting insert having enriched binder content at the surface and a reduced surface content of carbides having a B-1 type crystal structure. Other U.S. patents relating to enriched binder content at the surface include U.S. Pat. Nos. 4,610,931 to Nemeth et al; 4,548,786 to Yohe; and 4,497,874 to Hale et al. An extensive prior art summary is provided in the Hale patent.
Heretofore, the prior art has concentrated on adding titanium nitride or titanium carbonitride to aid the decomposition of the B-1 carbides near the surface during sintering and form a composition having an enriched binder content.